mkfs: don't trample the gid set in the protofile

mkfs: don't trample the gid set in the protofile

Catherine's recent changes to xfs/019 exposed a bug in how libxfs
handles setgid bits.  mkfs reads the desired gid in from the protofile,
but if the parent directory is setgid, it will override the user's
setting and (re)set the child's gid to the parent's gid.  Overriding
user settings is (probably) not the desired mode of operation, so create
a flag to struct cred to force the gid in the protofile.

It looks like this has been broken since ~2005.

Cc: Catherine Hoang <catherine.hoang@oracle.com>
Fixes: 9f064b7e ("Provide mkfs options to easily exercise all inheritable attributes, esp. the extsize allocator hint. Merge of master-melb:xfs-cmds:24370a by kenmcd.")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Catherine Hoang <catherine.hoang@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>